Active damping of high speed scanning probe microscope components

ABSTRACT

A technique for actively damping internal vibrations in a scanning probe microscope is disclosed. The excitation of various mechanical movements, including resonances, in the mechanical assembly of an SPM can adversely effect its performance, especially for high speed applications. An actuator is used to compensate for the movements. The actuator may operate in only the z direction, or may operate in other directions. The actuator(s) may be located at positions of antinodes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 11/678,018 filed Feb. 22, 2007, which claims priority to U.S.Provisional Application No. 60/776,385, filed on Feb. 23, 2006. Thedisclosure of the prior application is considered part of (and isincorporated by reference in) the disclosure of this application.

BACKGROUND

An atomic force microscope is a device used to produce images of surfacetopography (and other sample characteristics) based on informationobtained from rastering a sharp probe on the end of a cantileverrelative to the surface of a sample. Deflections of the cantilever, orchanges in its oscillation which are detected while rastering correspondto topographical (or other) features of the sample. Deflections orchanges in oscillation are typically detected by an optical leverarrangement whereby a light beam is directed onto a cantilever in thesame reference frame as the optical lever. The beam reflected from thecantilever illuminates a position sensitive detector (PSD). As thedeflection or oscillation of the cantilever changes, the position of thereflected spot on the PSD changes, causing a change in the output fromthe PSD. Changes in the deflection or oscillation of the cantilevertypically trigger a change in the vertical position of the cantileverbase relative to the sample, in order to maintain the deflection oroscillation at a constant pre-set value. This feedback generates theimage from the atomic force microscope, called an AFM image.

Atomic force microscopes can be operated in a number of differentimaging modes. In contact mode, the tip of the cantilever is in constantcontact with the sample surface. In oscillatory modes, the tip makes nocontact or only intermittent contact with the surface.

FIG. 1 shows one prior art approach to the use of actuators in an atomicforce microscope. A sample 1 is attached to a z-actuator 2. The base 7of a flexible cantilever 6 is attached to xy-actuator 8 which isattached to a head frame 9 (“xy” here represents that the actuator movesin the horizontal XY plane, and “z” represents that the actuator movesin the vertical direction, “X” and “Y” and “Z” being mutually orthogonaldirections). The xy-actuator 8, combined with the z-actuator 2, providesrelative motion between the probe 5 and the sample 1 in all threedimensions. The z-actuator 2 is supported by a structure 3 attached tothe frame 4 of the instrument. The cantilever 6 deflects in response tointeractions between the probe 5 and the sample 1. This deflection ismeasured by a PSD 10. The output of the PSD 10 is collected by thecontroller 11. Typically, the controller 11 performs some processing ofthe signal, extracting quantities such as the cantilever deflection,amplitude, phase or other parameters. These values are often displayedon a display device 12. Furthermore, the controller 11 can operate afeedback loop that in turn varies the relative position of the base 7 ofthe cantilever 6 and the sample 1 in response to sample characteristics.

Accurate characterization of a sample by an atomic force microscope isoften limited by the ability of the atomic force microscope to move thebase of the cantilever vertically in the Z direction relative to thesample surface at a rate sufficient to characterize the sampleaccurately while scanning in either the X or Y direction.

This movement rate is often expressed in terms of bandwidth. Thebandwidth required depends on the desired image size (in pixels) andacquisition rate of each pixel. Table 1 below shows the bandwidthrequired for various imaging scenarios. For example, completing a256.times.256 pixel image in one second requires a bandwidth of 131 kHz.TABLE-US-00001 TABLE 1 Closed loop bandwidths (BW) required for highspeed atomic force microscope imaging. Closed Loop bandwidths Required(kHz) Images/Second 128.sup.2 Pixels 256.sup.2 Pixels 512.sup.2 Pixels0.1 1.64 6.5 52.4 0.2 3.3 13 104 1 16.4 131 524 5 81.9 328 2620 10 164655 5240

In order to accurately measure the height of all features, both largeand small, on a sample surface, the z-actuator must have the ability toprovide relative motion between the base of the cantilever and samplesurface over a large range of heights, i.e., it must have large verticaltravel. The z-actuator can be a scanning tube in many conventionalAtomic force microscopes. In other microscopes, such as the AsylumResearch MFP-3D atomic force microscope, the z-actuator is a flexure.The parts must be large enough to move the cantilever up and downsufficiently to measure even the largest surface features.

The range of actuation of many actuators scales with the physicaldimensions of the device. This is certainly the case with piezoactuators. For example, in the case of commercially available piezostack actuators from TOKIN Incorporated, the maximum travel range isnominally 4.6 um, 9.1 um and 17.4 um for stacks having respectivelengths of 5 mm, 10 mm and 20 mm. Accordingly, a by-product ofincreasing the travel range, is that actuators become more massive.These more massive actuators move more slowly.

One way of characterizing the speed of actuator movement is the resonantfrequency of the actuator. The piezo stacks mentioned above haveresonant frequencies of roughly 261 kHz, 138 kHz and 69 kHzrespectively. It may be noted that the piezo material used in thesethree stacks is the same. The change in resonant frequency is primarilydue to the different sizes and therefore masses.

The quoted resonant frequencies are for the bare stacks. Attaching thesebare stacks to a support structure or incorporating them in a flexurewill substantially further reduce the resonant frequency. Furthermore,attaching any mass to the piezo will further reduce the resonantfrequency.

In practice, this means that the actuator may not be able to move eitherthe sample or the base of the cantilever rapidly enough to track thesurface accurately. This can lead to either the sample and/or probebeing damaged, or to the reproduction of the surface topography beingless accurate. In order to avoid these consequences, an atomic forcemicroscope operator will typically decrease the scan rate in the X and Ydirections until the z-actuator can accommodate the topographicalvariations in the sample.

Typically, an atomic force microscope operator begins by increasing thefeedback loop gain to increase the response of the z-actuator. However,at some point, the z-actuator will begin to resonate and that resonantmotion will create parasitic oscillations in the actuator supportstructure and even change the phase of the response of the actuator toinputs. These parasitic oscillations and phase changes reduce theperformance of the instrument and the quality of the images and otherdata produced.

The actuation scheme depicted in FIG. 1 is representative of a greatnumber of actuation schemes commonly used to provide relative movementbetween a tip and sample. This may provide a useful model for analyzingwhat happens when the atomic force microscope operator increases thefeedback loop gain to increase the response of the z-actuator.Increasing the feedback loop gain increases the extension of thez-actuator 2 in the vertical or Z direction. This increased extension,however, results in an increased reaction force on the support structure3.

FIG. 2 constitutes the lower-left segment of FIG. 1, showing this inmore detail. In FIG. 2, as in FIG. 1 the sample 1 is supported by thez-actuator 2. FIG. 2—Reaction Forces shows the z-actuator 2 extendingand moving the sample 1 a distance .DELTA.Z.sub.sam. This movementrequires a force F.sub.sam is exerted on the sample to causeacceleration of the sample. Newton's second law implies there is acorresponding reaction force F.sub.sup exerted on the support structure3. This reaction force will cause some deflection in the supportstructure, .DELTA.Z.sub.sup. Flexing of the support structure leads toringing, reduced motion of the sample (.DELTA.Z.sub.sam) and generallyreduced bandwidth of sample actuation.

One approach to overcoming the consequences of speeding up imageacquisition has been to allow the cantilever error signal to vary overthe scan range, but to keep the average value at a setpoint (Albrechtand Quate). With this approach, the job of the feedback loop is mademuch easier, allowing faster scanning. However, large variations in theerror parameter using this technique may have detrimental effects,including but not limited to tip dulling or damage and sample damage.

Another approach has been to use “nested” actuators. A large, relativelylong-range and slow actuator is used along with a small, short range butmuch faster actuator. This allows images to be obtained at higher speedsbecause the small fast actuator can accommodate small surface variationswhile the large actuator takes care of the gross height variations overthe entire XY scan range. One example of this approach is the zinc oxidepiezo actuators integrated into cantilevers by Sulchek et al. and Rogerset al. These actuated cantilevers, together with the typical actuatorscontrolling the distance between the cantilever base and the samplesurface, allows the effective distance between the base of thecantilever and the surface of the sample to be maintained constant atthe same time that the cantilever probe characterized the sample. Usingthese cantilevers, a bandwidth of 38 kHz has been demonstrated.

The actuated cantilever approach raises some difficulties. Combiningfast and slow feedback loops is not always trivial. Tuning two feedbackloops is significantly more time consuming and problematic than tuning asingle loop. The scan speed gain is often rather moderate consideringthe complexity required of the operator. Combining the final data toobtain an accurate characterization of the sample is also morecomplicated and prone to instrumentation errors and artifacts. Actuatedcantilevers are necessarily quite stiff, making imaging of soft samplesproblematic. Imaging in fluids, one of the strengths of the atomic forcemicroscope, is difficult to implement with actuated cantilevers becauseof the requirements for electrical contacts directly to the cantilever.Changing the sample to one with a mass different than the design valueof the actuated cantilever may seriously degrade its ability to overcomethe effects of fast image acquisition. Finally, even actuatedcantilevers have a resonance, a property which of course inducesparasitic oscillations and phase shifts and therefore reduces dataquality and leads to cantilever and/or sample damage.

Another approach to minimizing parasitic oscillations and phase shiftswhile speeding up atomic force microscope image acquisition is toconstruct the actuators in a recoilless, balanced arrangement wherethere is essentially no momentum transferred to the frame of theinstrument. Typically, this arrangement includes the use of additionaldamping material to correct for any small discrepancies in the design,construction or material properties of the balanced actuators. Thebalanced actuator approach has been used by Cleveland et al., Ando etal. and Massie. A weakness is that the system is an open loop system.If, for example, the actuated mass changes, as is common when the sampleis changed, or if the piezo sensitivity changes, as commonly happenswith age, the balancing will become less and less effective.

FIG. 3 depicts the balance actuator approach. As with FIG. 1, a sample 1is attached to a z-actuator 2 and the z-actuator is supported by asupport structure 3 attached to the frame of the instrument 4. In thiscase however, there is a secondary z-actuator 13 positioned below thesupport structure 3 and an optional mass 14 attached to the secondaryz-actuator 13. In addition, there can be a variable gain drive 15 forthe secondary actuator. The z-actuator 2, as well as the secondaryz-actuator 13 with variable gain drive 15 are driven with similar (orthe same) feedback signal. The gain provided by variable gain drive 15and the mass 14 are chosen so that the momentum transferred to thesupport structure 3 is substantially zero. Specifically, the forceexerted on the support structure 3 by the base of the z-actuator 2 isequal and opposite to that exerted by the base of the secondaryz-actuator 13.

Analogous operations can also be accomplished in a number of other ways,including using balanced flexures. See U.S. Pat. Nos. 6,459,088 B1 and6,323,483 B1 for a host of methods all with the goal of reducing themomentum transferred to the support to essentially zero. This method hasalso been used with a number of variations by the group of Ando.

The balanced actuator approach of FIG. 3 is an open-loop system in whichthe design is carefully focused on balancing opposed actuators andthereby avoiding the excitation of resonances in the support structure.An open loop system however presents disadvantages. Experimental resultsof systems such as that in FIG. 3 have shown that if the mass of thesample 1 changes, as commonly occurs in atomic force microscopeoperation, the balancing condition will no longer be met and momentumwill be transferred to the support structure 3 by the movements ofactuators 2 and 13. This in turn induces the very resonances theapproach seeks to avoid. Furthermore, the sensitivity of actuators canchange over time or in response to environmental conditions, and thistoo also introduces these problems. Finally, the open loop, balancedapproach requires very thorough manufacturing control.

The modifications in the balanced actuator approach recently proposed byAndo, attempt to overcome these disadvantages by including a model ofthe drive or “dummy” actuator (as they referred to it in their work) inthe drive of the otherwise open loop actuators 2 and 13. However thissolution has its own disadvantages when the behavior of either of theactuators begins to deviate from the “dummy” actuator. Ando'smodifications provide no mechanism to measure and automatically correctthe motion of the compensating system for the new actuator behavior.

A recent approach to the problem of speeding up image acquisitionwithout inducing parasitic oscillations and phase shifts is that ofKodera et al. They have proposed a method of damping the resonances of abalanced scanner based on Q-control ideas. Their method introduces a“mock scanner” into the drive circuit for the z-actuator which in turnallows the phase of the drive to be adjusted to reduce the amplitude ofthe actuator resonances. This approach suffers from the limitation thatthe behavior of the actuator has to be preprogrammed and if itscharacteristics change, the damping effects will no longer functionoptimally, if at all.

SUMMARY

Apparatus for actively damping the effects of inertial forces internalto the instrument created by the use of fast actuators in the operationof cantilever-based instruments, and methods for using such apparatusare disclosed herein.

The systems and techniques described herein provide a novel sampleand\or probe holder for cantilever-based instruments, particularlyAtomic force microscopes, that permits the probe to measure the heightof small surface features.

Systems and techniques provided herein allow for damping of the effectsof inertial forces internal to the instrument created by an actuator,and does so in a seamless manner that is preferentially invisible to anoperator and that is easier to manufacture than other. The damping isaccomplished in a judicious manner, based on intelligent interpretationof measurements of the reaction of the apparatus to forces created by anactuator and then compensation for those measured forces.

In one aspect, the techniques and systems for implementing thosetechniques provide a novel cantilever-based instrument that permits moreaccurate imaging of surface features at high scan rates.

In another aspect, these developments provide a cantilever-basedinstrument that can measure sample features at high scan rates byreducing parasitic oscillations in internal structure of the instrument.

In another aspect, the developments described here allow parasiticoscillations created by the fast z-actuator in a cantilever-basedinstrument to be damped out, allowing improved performance.

In yet another aspect of the operation mode and instrumentationdescribed here, it is possible to provide a fast actuator ofsufficiently low mass and/or small parasitic vibrations to allow its useon the lower end of a piezo-tube on a cantilever-based instrument.

The approach demonstrated here allows a cantilever-based instrument tobe improved, with fast actuation and operation in nested or parallelfeedback loops.

In another aspect, the scheme described here provides a cantilever-basedinstrument with the ability to cancel vibrations originating frominternal operations of the instrument or from external sources, shocks,disturbances or noise.

Another feature of the developments described here is that vibration atone or more locations in the cantilever-based instrument can be sensedand actively damped with actuators designed into the instrument.

A characteristic of the technique and physical implementation of itdiscussed here is that both the sensing and actuation can beincorporated into a single device so that the design and implementationof the active damping is simplified and cost and complexity is reduced.

This technique is useful for damping relative vibrations betweendifferent points of the instrument.

An advantage of the instrumentation and techniques described here isthat they provide, through active control of the damping, enhancedperformance that is substantially independent of the mass of the objectbeing actuated.

A further advantage of the provide information to the operator on howwell the active control of vibration is performing and to allow theoperator to modify the instrumental environment or operating parametersto optimize performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1—Actuated Sample depicts a prior art single z-axis actuator;

FIG. 2—Reaction Forces shows the behavior of a z-axis actuator and itssupport structure;

FIG. 3—Balanced Actuated Sample shows a prior art balanced actuator;

FIG. 4—Damped Actuated Sample shows one embodiment of the currentinvention;

FIG. 5—Compensated Forces shows a feedback procedure for activelydamping parasitic oscillations;

FIG. 6—Response Spectra shows measurements comparing prior arttechniques with the active damping method and apparatus;

FIG. 7—Self-sensing depicts an embodiment where the secondary actuatoris self-sensing;

FIG. 8—Higher Modes depicts a situation where modes other than thefundamental are being excited in the support structure; and

FIG. 9—Higher Mode Damping shows a technique for actively damping thefundamental and higher order modes in a support structure;

FIG. 10—Higher Mode Damping shows a technique for actively damping anorthogonal mode or modes in a support structure;

FIG. 11—Flexure Implementation 1 shows an embodiment where the sample isheld on a removable puck or holder; and

FIGS. 12A-12E—Compensated Z for Dimension type AFM show animplementation compatible with a scanning tip version of an atomic forcemicroscope.

DETAILED DESCRIPTION

Cantilever-based instruments include such instruments as atomic forcemicroscopes, molecular force probe instruments (1D or 3D),high-resolution profilometers and chemical or biological sensing probes.The embodiment describes atomic force microscopes (AFMs). Theembodiments encompass these devices as well as any other metrologyinstrument that can be used in nanoscale applications.

According to an embodiment, any mechanical oscillations in the atomicforce microscope support structure is measured. A damping force isapplied with the goal of preventing the parasitic oscillations fromdegrading performance. Active damping of the support structure enablesextremely accurate scanning of even the smallest surface features andeven at high scan speeds where conventional actuators suffer from poorperformance.

Embodiments can be used with cantilever-based instruments, that is, anapparatus having a probe for characterizing a sample. The apparatus mayhave an x-actuator, a y-actuator and a z actuator as in an atomic forcemicroscope. Actuators are commonly used in atomic force microscopes, forexample to raster the probe or to change the position of the cantileverbase relative to the sample surface. The actuators provide relativemovement between the probe and the sample. For different purposes anddifferent results, it may be useful to actuate the sample, or actuatethe tip or actuate some combination of both.

Sensors are also commonly used in Atomic force microscopes. They areused to detect movement of various components of the atomic forcemicroscope, including movement created by actuators.

For the purposes of the specification, unless otherwise specified, theterm “actuator” refers to a broad array of devices that convert inputsignals into physical motion, including piezo activated flexures, piezotubes, piezo stacks, blocks, bimorphs, unimorphs, linear motors,electrostrictive actuators, electrostatic motors, capacitive motors,voice coil actuators and magnetostrictive actuators. The term “positionsensor” or “sensor” refers to a device that converts a displacement,velocity or acceleration into an electrical signal, including capacitivesensors, inductive sensors (including eddy current sensors),differential transformers, variable inductance, optical interferometry,optical deflection detectors (referred to above as a PSDs), straingages, piezo sensors, magnetostrictive and electrostrictive sensors.

The apparatus may also have only a z-actuator such as in a profilometeror the Molecular Force Probe-1D product manufactured by Asylum Research.In these cantilever based instruments, these and other goals areachieved according by a probe (or sample) holder that includes a fastactuator assembly operated in a fast feedback loop and that alsoincludes an active feedback loop and secondary actuator that dampsparasitic oscillations in the assembly.

If the fast actuator assembly has low mass, and is therefore able todisplace the probe more rapidly, and is mounted on a larger, higher massconventional actuator, it could be operated in a fast feedback loop,either nested with the feedback loop of the conventional actuator or ina parallel feedback loop.

The fast actuator assembly comprises first and second fast actuatorssometimes referred to herein as a z-actuator and secondary (orcompensation or damping) z-actuator. There may alternatively beadditional actuators. The actuators can be arranged so that the fixedends are attached to a common support. If they are attached on oppositesides of the support, for example the top and the bottom, the top end ofthe top actuator and the bottom end of the bottom actuator are both freeto move. In an embodiment, the measurement probe, for example the probeof an atomic force microscope cantilever, is attached directly orthrough intermediate mounting to the bottom end of the bottom actuatorwhich is positioned closely the sample. A counterbalance mass may or maynot be attached to the top end of the top actuator. One or both of theactuators may also be preloaded in a flexure and arranged to cause theflexure to bend in response to actuation forces. Through this geometryand a feedback loop, the top and bottom actuators are arranged to movein opposite directions. The feedback loop causes motion in the supportstructure to be damped by the top (secondary) actuator.

In another embodiment, the sample is carried by the top actuator and thebottom actuator acts as the secondary actuator. The principal is thesame however. The feedback loop causes the bottom (secondary) actuatorto damp oscillations in the support structure.

For purposes of illustrating the damping embodiment, one can consider asingle axis system where the probe is being moved by a z-actuator, whichis the bottom actuator. An atomic force microscope may have interactionbetween the probe and the sample. A feedback loop may be used tomaintain this interaction at some preset level. This feedback loop willcontrol the z-actuator, which in turn will regulate the probe-sampledistance to maintain the interaction at the preset level with a presetbandwidth. Alternatively, the probe position may be controlled in amanner independent of a feedback loop, or with feedback that istriggered at a discrete event. Examples of the former includeforce-distance curves and oscillatory driving of the probe. Examples ofthe latter include triggered force distance curves and measurement modeswhere the probe position is positioned at a distance relative to sampletopography measured with a feedback loop.

When the z-actuator is energized, the probe will move vertically to adesired position. This motion will necessarily impart a reaction forceon the support structure which, by Newton's second law will inducemotion in the support structure. This motion is detected with anothersensor which activates a feedback loop controlling a secondary actuator.The feedback loop operates to damp this measured motion in the supportstructure, thereby reducing parasitic oscillations.

An embodiment is shown in FIG. 4. Sample 1 is attached to a z-actuator 2which is supported by a support structure 3 attached to the frame of theinstrument 4. Sensor 16 attached to the support structure 3 outputs asignal corresponding to momentum-causing action on the support structure3 to the controller 111. The actions on the support structure can bemovement, acceleration, position and/or velocity provision, for example.The controller 111 uses this signal in a feedback loop that drives thesecondary z-actuator 13 to actively damp the vibrations of the supportstructure 3.

In one embodiment, the secondary z-actuator 13 has a small mass 14attached to its end which acts as a “test mass” to improve thesensitivity of the sensor 16 to reaction forces in the support structure3.

When the sensor 16 measures movement in the support structure 3, thecontroller 111 operates as feedback electronics to move the secondaryactuator 13 in a manner to damp the movement in the support structure 3.

FIG. 5 is a flow chart showing the sequence of events beginning with thesensing of movement in the support structure 13 and ending with thedamping of that movement. This flowchart may be executed by thecontroller 111, or via dedicated control circuitry.

FIG. 5 illustrates the input 500 from the sensor 16 being analyzed at510. 520 determines whether the sensor value indicates that there ismotion. If not, then no action is taken, and the flow continues.However, if motion is found at 520, than a compensation calculation iscarried out at 530, producing an output 540 to secondary z actuator 13.This compensation produces a value to the actuator 13 that damps themotion.

The active damping approach of the embodiment is quite different thanthe “balanced” actuator described by U.S. Pat. Nos. 6,459,088 B1 and6,323,483 B1 in that the steps of measuring the induced motion of thesupport structure and of actively damping this motion. The prior artbalanced actuator is designed such that the momentum transfer to thesupport structure is “substantially zero”. In the embodiment, momentumtransfer itself is stopped. If there is substantially zero motion in thesupport structure, then the sensor does not measure any motion and thefeedback loop does not energize the second compensating z-actuator.

FIG. 6 shows exemplary measured differences between the approachesdiagrammed in FIG. 1, FIG. 3 and FIG. 4. For these measurements, thecantilever probe 5 is brought into contact with the sample 1, in thiscase freshly cleaved mica. The deflection signal is monitored as thez-actuator 2 was excited at a range of frequencies (or “chirped”). Thecurves in FIG. 6 show the frequency dependent responses of the probe 5being excited by the chirped z-actuator 2. Specifically, curve 17 is theresponse of the PSD 10 when the probe 5 is in contact with the sample 1and when the z-actuator 2 was excited. FIG. 6 shows large peaks in theresponse amplitude which correspond to resonances in the supportstructure 3. These resonances are being driven by the reaction forcesbetween the support structure 3 and the base of the z-actuator 2. Formany positioning tasks, including such tasks with Atomic forcemicroscopes, this resonant motion in the support structure may be highlyundesirable.

The curve with the open squares 18 a shows the response amplitude of abalanced actuator as described in U.S. Pat. Nos. 6,459,088 B1 or6,323,483 B1, depicted in FIG. 6. As expected from the results describedin these patents, the response amplitude 18 a of the resonant peaks hasbeen significantly reduced, implying that the momentum transfer to thesupport structure 3 has been reduced, though it is not identically zero.To obtain a curve closer to zero, it is necessary to manufacture thebalanced actuator with higher tolerances, such that their positions andmasses and motions cancel each other out more perfectly. This tolerancerequirement is a disadvantage of the prior art balanced actuatorapproach. Nevertheless, the curve 18 a is desirable in that the motionof the support structure is reduced.

A disadvantage of this prior art approach becomes apparent when a 3 grammass is introduced at the sample position. There is a significant changein the frequency response, as shown in the resulting response amplitudecurve 18 b (closed squares). This curve shows a much larger response,with the now unbalanced actuators 2 and 13 driving the support structure3 to larger amplitudes at the resonances. This behavior is undesirableand significant. Switching from one sample (or probe) to another, withthe samples (or probes) possessing different masses, is common in workwith Atomic force microscopes.

An embodiment, which actively damps the sensed motion of the supportstructure 3 with different samples is shown in curves 19 a and 19 b.Curve 19 a shows the response amplitude with the same sample as curve 18a, the balanced actuator curve. As may be seen, the response amplitudeis somewhat larger than that derived from the balanced actuatorapproach. This is because this embodiment uses the sensor 16 to measuresome motion of the support structure 3 before the compensating feedbackloop is activated. Nevertheless, the amplitude is still much reducedfrom the single actuator approach (curve 17). When a 3 gram mass isadded, the resulting curve 19 b is almost indistinguishable from theoriginal curve 19 a, and is better than the equivalent balanced actuatorcurve (curve 18 b).

This demonstrates that the active damping approach can successfullyhandle changes in the actuated mass. In the same manner, the activemethod described here will automatically account for changes in actuatorsensitivity over time.

A problem with piezo actuators is that their sensitivity can depend ontime, temperature and other environmental factors. Active measurementand feedback compensation of the support structure motion willautomatically account for this behavior, yielding a system that is veryrobust.

Sensor noise will affect the efficiency of the active damping approach.Lower noise sensors or multiple sensors placed at critical positions,may give better performance than the balanced actuator approach, evenwhen great care is taken in the manufacture of the balanced actuator.There are almost always practical, real-world effects in the manufactureof these sorts of devices that make it difficult to balance the actuatorwith precision in all environments. However, this is a strong point ofthe active damping design. Manufacturing tolerances do not need to be asstringent. Any asymmetry in the construction will be measured by thesensor or sensors. The feedback loop/secondary actuator combination willcompensate for the imperfection.

For some purposes, it may be desirable to simplify the design ofinstruments employing the active damping approach. One embodiment uses aself-sensing actuator as is depicted in FIG. 7. Here, the sensor 16 andsecondary z-actuator 13 of FIG. 4 are replaced with a singledual-purpose device 20. This device 20 acts as both a sensor andsecondary z-actuator. Device 20 could be, for example, a piezo stackwith external circuitry that both detects the motion of the piezo andresponds by controlling that motion. Other technologies could also beemployed for this purpose as is well known to those skilled in the art.

The vibrations in support structure 3 sensed by the dual-purpose device20 and the appropriate response can be controlled by compensatingelectronics 21, which can be external or internal to the controller 111,but are shown here as external.

Another embodiment uses the controller 111 to control the feedback loopand compensate for the vibrations.

A small mass 114 may optionally be attached to the end of thedual-purpose device 20 as a “test mass” to improve the sensitivity ofthe device to reaction forces in the support structure 3.

The use of a flexure or clamped end on the dual-purpose device 20 mayimprove its performance. When the inventors used a piezo stack as thesensor, and attached a small mass to the end as a “test mass” to improvethe sensitivity of the stack to the reaction forces in the supportstructure 3, the result was a peak signal of 20 mV at one of the supportstructure resonances of 8 kHz. When the same piezo stack was insteadclamped and pre-loaded against the support structure, the peak signalincreased to over 103 mV, a greater than 6.times. improvement wasobtained. Larger signals are advantageous for constructing a moreaccurate and robust feedback loop as is well known to those skilled inthe art.

For stability and selectivity reasons, it may be advantageous to controlthe bandwidth of the feedback loop. For example, if the primaryobjective is to control a particular support structure resonance orrange of resonances, a feedback loop with a narrow bandwidth surroundingthose resonances may be preferable to a wide band feedback loop. Also,depending on the mechanical response of the support structure and sensorand the electronic response of the sensor conditioning, there may befrequency dependent phase shifts in the system that make wide bandfeedback difficult or impossible. In this case, choosing the bandwidthmay improve the performance of the instrument. Compensating at specificfrequencies allows the feedback loop to be simplified and to be morerobust. There are numerous means of limiting the feedback compensationto a specific range of frequencies including the use of analog and/ordigital high-pass, low-pass, or band-pass filters. It may also beadvantageous to have more than one feedback loop, where one loop isoptimized to damp resonances in a certain range and a second loop isoptimized to damp resonances in a different range. Additional feedbackloops that are specific to different frequency ranges could also beused.

It may also prove advantageous for separate feedback loops to be usedfor each sensor/actuator combination. In this case, the frequency rangesof the feedback loops may or may not overlap. However, the primary jobof a given feedback loop is to operate a given sensor/actuatorcombination. Information from others sensors or actuators could be usedin the implementation of these specific feedback loops as well, since itis possible and perhaps even likely there will be mechanical couplingbetween the various sensors and actuators.

In the prototypes that have been constructed, it has been advantageousto have the resonant frequency of the support motion sensing structurebe above that of the support structure itself. This allows the use of amore simple feedback loop to damp the support vibrations. If this is notthe case, more complicated feedback schemes such as H-infinitytechniques can be implemented to control the support motion even throughone or more resonances of the sensor structure.

Typical support structures are often much more complicated than thesimple geometry shown in FIG. 4 and FIG. 7. There are many engineeringrequirements that affect performance, manufacturability and quality thatneed to be evaluated when designing precision instrumentation of thetype considered here. Moreover, understanding the vibrationalcharacteristics of the instrument can be difficult. Thus, for any givensupport structure, it is useful to evaluate the various vibrationalmodes that compromise the performance of the instrument and then addactive damping to various locations to control it.

There are a number of techniques for evaluating unwanted vibrationalmodes in an instrument during the design phase. These include computermodeling of the structure and measuring movement with a laser vibrometeror other instrument. After such evaluation, the active damping approachof the current invention can be used to selectively damp thosevibrations. One embodiment using this approach to address suchvibrations is shown in FIG. 8. The FIG. 8 embodiment uses a payload 22in the place of a sample. The payload is moved in the vertical directionby a z-actuator 2. Unlike the embodiment depicted in FIG. 1, thisembodiment attaches the support structure 23 is attached to the frame ofthe instrument 4 in two places 88 and 89. A motion sensor 816 attachedto the support structure 23 detects vertical motion of the supportstructure 23, sends a signal to a controller 811. Controller 811 in turnsends a signal to damp out the detected motion through the use of acompensation z-actuator 813. In one manifestation, the compensationz-actuator 813 has an additional mass 814 attached to its end which actsas a “test mass” to improve the sensitivity of the motion sensor 816 toreaction forces in the support structure 23. However, the payload 22might not be symmetric about the vertical axis 24. When the compensationz-actuator 813 is moved, it causes a lateral (torsional) reaction force(torque) to be exerted on the support structure 23. This in turn excitesa second mode oscillation, indicated by the dashed structures 25. Thesecond mode oscillations of the support structure 25 have anti-nodes attwo particular positions 26 and 27.

The anti-nodes may be the best place to position compensationz-actuators to damp the second mode motion. FIG. 9 illustrates anembodiment using this kind of damping. As with FIG. 8, the parasiticoscillations of the support structure 23 along the vertical axis 24 aredamped out by a compensation z-actuator 13. In addition, the embodimentof FIG. 9 includes additional sensors 28 and 29 to measure the motion attheir respective positions of second mode oscillations, depicted atlocations 26 and 27 in FIG. 8. The signals corresponding to thesemotions are output to the controller 911 which uses this information tocontrol compensation z-actuators 30 and 32, each with optional addedmasses 31 and 33 to improve the sensitivity of the motion sensors 16, 28and 29 to reaction forces in the support structure 23.

While the controller 11 depicted in FIG. 9 is a central unit, the samefunctionality could be accomplished with distributed controllers, eachcontrolling one or more of the sensor/compensation z-actuatorcombinations. By controlling the motions of compensation z-actuators 30and 32, the higher mode oscillations can be damped. Similar performancecould be obtained using dual purpose self-sensing actuator, positionedat the anti-nodes 26 and 27. This approach can be extended to any numberof vibrational modes of the support structure 23.

Another embodiment of damping other modes is shown in FIG. 10. In thisembodiment, the swaying or torsional motion illustrated in FIG. 8 can becompensated with a sensor 34 and actuator 35 arranged with a componentorthogonal to the primary axis 1024. In this Figure, the compensation isaccomplished with an actuator 34 and an added mass 35. An additionalsensor/actuator combination could be deployed parallel and/orperpendicular to the axis 1024.

FIG. 11 shows an embodiment where the sample 1 is held on a removablepuck or holder 1101. This holder 1101 is optionally fixed to the sampleusing a magnet 1102 or other mechanism such as the stub clamps of thetype used in scanning electron microscope sample stubs of the type wellknown to those in the art. The sample puck is fixed to a flexureassembly 1103 that contains a primary actuator 1104 similar to the typediscussed above. When directed to by the control electronics 11, thisprimary actuator exerts a force on a portion of the flexure assemblythat causes flexure members 1105 to deflect, thus moving the samplealong the force axis. A sensor 1106A is fixed near the base of theprimary actuator 1104. The output of this sensor measures thedisplacement, velocity or acceleration of the portion of the flexureassembly 1103 where the primary actuator 1104 is attached 1108. Inanother embodiment, a different sensor 1106B measures the strain inducedbetween the flexure assembly 1103 and the attachment region 1108. Thissensor signal in turn is used by the control electronics 11 to drive thecompensating or secondary actuator 1107. The job of this actuator is todamp out vibrations that the sensor 1106A or 1106B measures in the basefixture of the flexure assembly 1103. As with other embodiments, thesensor 1106A or 1106B could actually be multiple sensors. The secondaryor compensating actuator 1107 can be a piezo element alone or couldcarry an extra mass of the type discussed elsewhere in this document.Actuator 1107 can also be preloaded with a fixed or adjustable forceimplemented with a preload screw 1109 or other device.

The secondary actuator 1107 can also be a non-piezo actuator. Anyactuator that is capable of damping the motion measured by the sensor(s)1106A and/or 1106B can be used. In one embodiment, the entire flexureassembly 1103-1109 is responsible for moving the sample in the verticalor z-direction. It is in turn coupled 1110 to an actuator 8 thatprovides relative motion between the sample 1 and the probe tip 5.

FIGS. 12A-12E show an implementation compatible with a scanning tipversion of an atomic force microscope. This includes the commerciallyavailable “Dimension” series, Dimension VX series and “Metrology” seriesmicroscopes available from Veeco instruments (Woodbury, N.Y.). Thismicroscope has a removable probe holder and is described for example inU.S. Pat. Nos. 5,714,682 and 6,861,649 and others. The compensated probeholders described in FIG. 12A-12E are designed to replace the probeholders described in the above patents and marketed under the Dimension,Dimension VX and similar trade names from Veeco Instruments.

FIG. 12A shows a view of a probe holder sub-assembly where thecantilever 612 is pointed substantially out of the plane of the paper.FIG. 12B shows the same sub-assembly rotated 90 degrees about the z-axis1208 of the sub-assembly. A probe 512 attached to a chip 7 is mounted ina pocket 1203. This pocket is in turn attached to a primary actuator1204. This is attached to a primary central support 1205.

In views 1201 and 1210, the damping of the central support 1205 inresponse to the motion of the primary actuator 1204 is accomplished witha combined detector/actuator 1206 of the type discussed above. Thisactuator has an optional reaction mass 1207 attached to it. Instead of areaction mass, it is possible and in some cases desirable to use aflexure design similar to the embodiments discussed above for thecompensation sub-assembly. In this embodiment, the same element 1206 isused to both detect the motion of the central support and to compensatefor that motion. FIG. 12C shows a different embodiment where themeasurement sensor 1221 and compensation actuator 1222 are separateentities. As with the embodiments discussed above, the signal that ismeasured, either by 1206 or by 1221, is used in a feedback loop tocontrol the motion of the compensation actuator, either 1206 or 1222.

FIGS. 12D and 12E show the sub-assemblies of 1201 and 1220 attached to abase or holder 1231 which defines a plurality of sockets, apertures orpins 1232 designed to allow the entire assembly to be operativelyconnected to the z-actuator in the Dimension or similar head.

As mentioned above, U.S. Pat. Nos. 6,459,088 B1 and 6,323,483 B1 show ahost of methods all with the goal of reducing the momentum transferredto the support to essentially zero. This method has also been used witha number of variations by the group of Ando. The multiple embodimentsdescribed herein can also use the general structures described in thosereferences, with the key exception that the embodiments may require anadditional sensor for measuring the performance of the feedback dampingfunction. If self-sensing actuators are used, the actuation assemblydescribed in this document may appear to be quite similar to theassemblies discussed in the above references. However, the way theperformance is achieved and the functionality is very different.

In one approach to positioning the compensation z-actuators and sensors,the locations are “designed” into the instrument. In another approach,the instrument is first designed and then the modes are experimentallymeasured, with a motion measurement device, such as a laser vibrometeror a sensor(s) of the type that could be used in the current invention.After the vibrational modes have been measured, active damping sensorsand actuators are then deployed into positions to improve or optimizethe device performance. For these sorts of applications, self-sensingactuators may be particularly useful.

Although only a few embodiments have been disclosed in detail above,other embodiments are possible and the inventor(s) intend these to beencompassed within this specification. The specification describesspecific examples to accomplish a more general goal that may beaccomplished in another way. This disclosure is intended to beexemplary, and the claims are intended to cover any modification oralternative which might be predictable to a person having ordinary skillin the art. For example, other applications beyond the measurement fieldare contemplated.

Also, the inventor(s) intend that only those claims which use the words“means for” are intended to be interpreted under 35 USC 112, sixthparagraph. Moreover, no limitations from the specification are intendedto be read into any claims, unless those limitations are expresslyincluded in the claims. The computers described herein may be any kindof computer, either general purpose, or some specific purpose computersuch as a workstation. The controller described herein may be a Pentiumclass computer, running Windows XP or Linux, or may be a Macintoshcomputer. The computer may also be a handheld computer, such as a PDA,cellphone, or laptop.

The programs may be written in C, or Java, Brew or any other programminglanguage. The programs may be resident on a storage medium, e.g.,magnetic or optical, e.g. the computer hard drive, a removable disk ormedia such as a memory stick or SD media, or other removable medium. Theprograms may also be run over a network, for example, with a server orother machine sending signals to the local machine, which allows thelocal machine to carry out the operations described herein.

Where a specific numerical value is mentioned herein, it should beconsidered that the value may be increased or decreased by 20%, whilestill staying within the teachings of the present application, unlesssome different range is specifically mentioned.

What is claimed is:
 1. A method, comprising: maintaining a feedback loopthat maintains a distance relationship between a sample being measuredand a support in a sensing system, said feedback loop operating tomaintain said distance relationship between said sensing system and saidsample that is supported by the support, said maintaining using a firstactuator; measuring momentum-causing actions that are induced to thesupport structure by actions of components of said sensing system otherthan said first actuator; and using a second actuator, separate from thefirst actuator, to compensate said momentum-causing actions to therebydamp said momentum-causing actions.
 2. A method as in claim 1, whereinsaid system includes components of an atomic force microscope and wheresaid components other than the first actuator are other components ofsaid atomic force microscope.
 3. A method as in claim 2 wherein saidmeasuring comprises measuring in a constant contact type atomic forcemicroscope.
 4. A method as in claim 2, wherein said measuring comprisesmeasuring in a scanning tip atomic force microscope.
 5. A method as inclaim 1, wherein said sensing system includes a measuring cantilever. 6.A method as in claim 1, further comprising using a controller todetermine a compensation based on said measuring, and to use saidcompensation to produce an output to said second actuator.
 7. A methodas in claim 1, wherein said measuring and said using are carried outwith separate structures.
 8. A method as in claim 1, wherein saidmeasuring and said using are carried out with a single structure thatproduces an output signal based on motion thereof, and which can movebased on an applied signal.
 9. A method as in claim 1, wherein saidmeasuring comprises determining positions of specified movements by saidsupport, and compensating movements at said positions of said positions.10. A method as in claim 9, wherein said positions of specifiedmovements are anti-nodes.
 11. A method as in claim 1 wherein said usingthe second actuator comprises using a third actuator that is part of thesecond actuator and that is oriented in a first direction relative to adirection of motion, and using a fourth actuator that is part of thesecond actuator and that is oriented orthogonal to said third actuator.12. A method as in claim 1, further comprising placing said sample on aremovable holder, and wherein said measuring comprises measuring motionon the removable holder.
 13. A method as in claim 1, further comprisingfurther compensating said actuator using a weight.
 14. A method as inclaim 1, further comprising attaching a test mass to said secondactuator to improve sensitivity of the second actuator to the momentumcausing actions.
 15. A method as in claim 1, wherein saidmomentum-causing actions include one or more of movement, positioning,velocity provision and/or acceleration.
 16. A method as in claim 1,further comprising placing said sample on a removable holder, andwherein said measuring comprises measuring motion on the removableholder.